Avionic functions installed in equipment located in a protected area, in a cabin or alongside passengers in an aircraft performing various functions necessary for the accomplishment of a flight, communicate with one another via a network.
Some of these functions being critical, such as, for example, the braking system or the flight management system (FMS), the network allowing them to exchange messages must meet certain determinism constraints and, in particular, guarantee a maximum latency for communications.
For reasons of performance, cost and limitation of aeroplane cabling, choices have tended towards switched networks and the use of the Ethernet standard.
Conventional Ethernet networks do not provide this determinism. In particular, they do not guarantee maximum latency for communications. For this reason, in the domain of avionics, switched Ethernet networks are used, to which a network overlay is added. This overlay is described in the standard ARINC 664.
The operation of this overlay is based on a limitation of the bandwidth and on a distribution of the messages over the network in such a way that it is possible to determine a maximum latency from one end of the network to the other.
The network comprises routers responsible for the transmission of the messages. These routers receive messages via input ports and transmit them via output ports.
On a switched network, regardless of the standard used (Ethernet or other), latency is created mainly by the router output ports. In fact, each of these output ports comprises a queue in which the messages are placed one after the other before being transmitted.
Once a maximum message size and a minimum delay between messages are defined, it is possible to determine a maximum latency for each of the output ports of each router of the network.
The maximum latency from one end of the network to the other therefore depends mainly on the following parameters:                the maximum size of the messages (also referred to as the maximum frame size),        the minimum delay between two messages (also referred to as the minimum delay between frames or BAG for Bandwidth Gap),        the cumulated latency created by the routers on a given path.        
According to the prior art, in order to provide a means of segregation between data flows and to guarantee the determinism of these networks, virtual links are defined in such a way that the characteristics essential to the rating of the system are guaranteed over the entire network. A virtual link is a communication channel for which it is possible to define the demonstrated qualities of service (typically bandwidth and latency). In the current prior art relating to switched networks, a virtual link must include the characteristics of spacing (BAG), message size (Maximum Frame Size) and the routing used to transport the messages (links travelled).
Once the virtual links have been defined, these essential characteristics, in particular bandwidth and latency, are guaranteed for each virtual link independently of the use that is made of the other virtual links:                The bandwidth of a virtual link is available regardless of the use that is made of the other virtual links.        Latency in the worst case is guaranteed regardless of the use that is made of the other virtual links.        
The definition of a virtual link includes the following parameters:                the minimum time between two messages (BAG),        the maximum size of a message (MaxFrameSize), which can be transmitted on this virtual link,        the links used, i.e. the physical path corresponding to this virtual link, allowing the routers and the ports of the routers used to be identified,        the priority allowing the latency to be optimised for the most critical data exchanges.        
On the basis of these data, the bandwidth and latency of a virtual link can be determined in the following manner:
The available bandwidth is equal to the ratio between the maximum size of a message and the minimum time between two messages (MaxFrameSize/BAG).
Latency (from one end of the network to the other) is determined using a specific algorithm approved for the avionic domain and taking account of all the virtual links defined over the network.
These two characteristics linked to the virtual links allow the determinism of the network to be guaranteed. However, as seen above, the latency calculation can only be carried out when all the virtual links are defined. This causes the following problems during the definition of a network:                The virtual links can be defined only when the corresponding data flows are sufficiently characterized, which happens quite late in the definition of the network.        The knowledge of the links used is required for the definition of the virtual links and therefore for the latency calculation. This is possible only when the architecture of the network is determined.        The determinism guarantee can only be provided when all the virtual links are defined.        
These constraints do not allow the definition of a network with which latency constraints are associated to be validated in an early manner. In fact, all the VLs must be defined in order to determine whether the latency constraints are respected by the network.